The present invention generally relates to the technical field of a power supply device, and in particular, relates to protection of a semiconductor switch of a high frequency amplifying circuit.
In a high frequency power source for plasma generation, an unexpected change of the plasma density due to abrupt abnormal electric discharge or due to a pulse outputting operation of the high frequency power source may occur in the plasma serving as a load. When this happens, the load impedance seen from the high frequency power source may instantaneously fluctuate to increase the output current or voltage of a high frequency amplifying circuit. Therefore, frequent occurrence of this phenomenon may cause an electric stress on, or may cause breakage of, the high frequency amplifying circuit or an internal element of a high frequency output circuit.
For example, in abnormal electric discharge occurring during plasma processing, a high-frequency output is stopped in several hundreds of nanoseconds after the occurrence of abnormal electric discharge. The plasma energy is then dissipated, and the high frequency power is turned on again thereafter. In this method, the plasma returns from a misfire state to a steady state in several microseconds using several tens of microseconds. If re-ignition of plasma fails, the misfire state continues and the plasma will not return to the steady state. The impedance of the high frequency amplifying circuit would mismatch with the load, and as a result, power loss would occur.
In order to suppress the power loss of the high frequency amplifying circuit, drooping characteristics of the high frequency amplifying circuit may be controlled under the feedback control of the output power. However, in many cases, the response is slow and thus the output power will drop through several hundreds of microseconds to several milliseconds. Therefore, the protection of each element of the high frequency amplifying circuit will not work under the feedback control during the decrease of output power.
If the impedance from the high frequency amplifying circuit to the load is significantly lower than usual, the high frequency amplifying circuit will output the energy several times larger than the normal energy. Therefore, a semiconductor switch of the high frequency amplifying circuit will cause power loss, overvoltage, and/or overcurrent. In attempting to design in consideration of the overvoltage and overcurrent due to the fluctuation of a load impedance, the designer of a high frequency power source has to provide a semiconductor switch having the rating several times higher than the normal semiconductor switch and design a circuit having a high withstanding voltage. Thus, reducing size and reducing component cost are inhibited.